In the production of semiconductor integrated circuits, lithographic exposure tools for reductively projecting and transferring a fine circuit pattern drawn in a photomask onto a wafer are extensively used. With the trend toward higher degrees of integration and higher functions in circuits, the circuits are becoming finer and the lithographic exposure tools have come to be required to form a high-resolution circuit pattern image on a wafer surface while attaining a large focal depth. The wavelengths of exposure light sources are becoming shorter. KrF excimer lasers (wavelength, 248 nm) and ArF excimer lasers (wavelength, 193 nm) are being used as exposure light sources in place of the g-line (wavelength, 436 nm) and i-line (wavelength, 365 nm) heretofore in use.
Photomask substrates mainly used for lithographic exposure tools employing such exposure light sources are ones made of a synthetic quartz glass, because synthetic quartz glasses have advantages, for example, that they have excellent transparency to light in a wide range of from the near infrared region to the ultraviolet region, have an extremely low coefficient of thermal expansion, and can be processed relatively easily. Photomask substrates for, e.g., ArF excimer lasers are required to have a surface flatness of about 0.5 μm, a parallelism of about 5 μm, and a birefringence of about 4 to 10 nm/cm besides resistance to ArF excimer laser light.
Recently, the technique of immersion exposure is known in which exposure with a lithographic exposure tool is conducted while filling the space between the projection lens of the lithographic exposure tool and the wafer with a liquid in order to attain a higher resolution with an ArF excimer laser. The shorter the exposure light wavelength and the larger the NA (numerical aperture) of the projection lens, the higher the resolution for the lithographic exposure tool becomes. The resolution can be represented by the following expressions.Resolution=[k (process coefficient)×λ(exposure light wavelength)]/NANA=n×sin θIn the expressions, n indicates the refractive index of the medium through which the exposure light passes. In exposure techniques heretofore in use, n is 1.0 because the medium is the air. In this immersion exposure, however, pure water, which has an n of 1.44, is used as the medium and the lithographic exposure tool can hence attain an even higher resolution.
Furthermore, the polarized illumination technique is known in which polarized lights which exert an adverse influence on resolution are diminished to thereby heighten image-forming contrast and improve resolution, in contrast to the exposure techniques heretofore in use which employ an exposure light composed of random polarized lights having various polarization directions.
The photomasks for use in such immersion exposure technique and/or polarized illumination technique are required to have low birefringence so as not to disorder the polarization of the exposure light which passes therethrough. A photomask substrate having a birefringence reduced to 2 nm/cm or less has hence been proposed (see, for example, patent document 1).
Patent Document 1: JP-T-2003-515192
In patent document 1, the birefringence of the photomask substrate is specified. This birefringence of the photomask substrate is mainly attributable to a residual strain in the synthetic quartz glass used as the photomask substrate. However, in the case of a mask blank comprising a photomask substrate and a light-shielding film laminated thereon, the birefringence thereof is attributable also to the stress imposed by the light-shielding film laminated on a surface of the photomask substrate. It is therefor necessary that this film stress should be taken into account in regulating the birefringence of a mask blank comprising a photomask substrate and a light-shielding film laminated on a surface of the substrate.